Heretofore, in the semiconductor industry, a photolithography method employing visible light or ultraviolet light has been used as a technique to transfer a fine pattern required to form an integrated circuit with a fine pattern on e.g. a silicon substrate. However, the conventional photolithography method has come close to its limit, while miniaturization of semiconductor devices is being accelerated. In the case of the photolithography method, the resolution limit of a pattern is about ½ of the exposure wavelength. Even if an immersion method is employed, the resolution limit is said to be about ¼ of the exposure wavelength, and even if an immersion method of ArF laser (wavelength: 193 nm) is employed, about 45 nm is presumed to be the limit. Under the circumstances, as an exposure technique for the next generation employing an exposure wavelength shorter than 45 nm, EUV lithography (hereinafter, in this specification, “EUV lithography” is referred to simply as “EUVL”) is expected to be prospective, which is an exposure technique employing EUV light having a wavelength further shorter than ArF laser. In this specification, EUV light is meant for a light ray having a wavelength within a soft X-ray region or within a vacuum ultraviolet region, specifically for a light ray having a wavelength of from about 10 to 20 nm, particularly about 13.5 nm±0.3 nm (from about 13.2 to 13.8 nm).
EUV light is likely to be absorbed by all kinds of substances, and the refractive index of substances at such a wavelength is close to 1, whereby it is not possible to use a conventional dioptric system like photolithography employing visible light or ultraviolet light. Therefore, in EUV lithography, a catoptric system, i.e. a combination of a reflective photomask and a mirror, is employed.
A mask blank is a stacked member before pattering, to be employed for the production of a photomask. In the case of an EUV mask blank, it has a structure wherein a reflective layer to reflect EUV light and an absorber layer to absorb EUV light, are formed in this order on a substrate made of e.g. glass. As the reflective layer, it is common to use a multilayer reflective film having a low refractive index film and a high refractive index film, alternately stacked to have the light reflectivity improved when its surface is irradiated with EUV light. As a low refractive index film in the multilayer reflective film, a molybdenum (Mo) layer is usually employed, and as a high refractive index film, a silicon (Si) layer is usually employed.
For the absorber layer, a material having a high absorption coefficient to EUV light, specifically e.g. a material containing chromium (Cr) or tantalum (Ta) as the main component, is used.
In a case where a multilayer reflective film is used as a reflective layer of an EUV mask blank, it is necessary to increase the film density of each layer in the multilayer reflective film in order to increase the light ray reflectivity at the time of applying EUV light, and the multilayer reflective film necessarily has a high film stress (namely high compression stress).
If such a high film stress is applied on a substrate, the substrate may be deformed. As a substrate for an EUV mask blank, a substrate made of low expansion glass is usually used, whereby the deformation of the substrate due to the film stress is slight and was not problematic in the past.
However, along with a demand for a fine pattern, the slight deformation of a substrate (namely, the deformation of the substrate due to the application of the film stress) which was not considered as a problem in the past has become problematic. For example, if a deformation larger than a specific size is present in the substrate for an EUV mask blank, specifically, in a case of a 152 mm×152 mm substrate which is usually used for producing an EUV mask blank, if the degree of warpage of the substrate exceeds 0.6 μm, the positioning accuracy of a pattern at the time of patterning the EUV mask blank may deteriorate. Further, if warpage of such a degree results, at the time of transferring a pattern by using a reflective mask produced from the EUV mask blank, the mispositioning of the pattern or pattern defects may result.
Further, it has been confirmed in Patent Document 1 that in an EUV mask blank after production, due to thermal factors, the film stress in a multilayer reflective film changes with time. Further, it has been confirmed in Patent Document 1 that due to thermal factors in a step of cleaning an EUV mask blank or in a baking step after film formation of a resist film carried out for producing a reflective mask from the EUV mask blank, the film stress in a multilayer reflective film changes.
It is disclosed in Patent Document 1 that such a change of the film stress is caused due to extremely small mixing at interfaces of the respective layers constituting the multilayer reflective film. The degree of such a change is at such a level that cannot be detected by measuring the length of a period by x-ray reflectivity film thickness measurement. However, by such a change, a peak wavelength of the reflectivity of the multilayer reflective film (namely, the wavelength at the peak reflectivity of the multilayer reflective film) changes at a level of 0.01 nm. Since EUV light is light having a very short wavelength, the change in the state of the multilayer reflective film very sensitively influences its wavelength property and reflective properties.
Further, since light having a specific short wavelength range is used for an EUV lithography, the influence of the wavelength shift is large, and the shift of the peak wavelength of the reflectivity causes mismatch with a mirror of an exposure system used at the time of transferring a pattern. Thus, the peak wavelength has to be accurately controlled. Further, due to the shift of the peak wavelength, the reflectivity of the multilayer reflective film is lowered. As described above, the change with time of the stress of the multilayer reflective film causes various problems such that the flatness of a substrate is changed, when a mask is practically used.
In Patent Document 1, in order to solve the above problems, at the time of forming a multilayer reflective film and/or after forming the multilayer reflective film, a substrate on which the multilayer reflective film is formed is subjected to heating treatment. Thus, by suppressing the progress of mixing at interfaces of the respective layers which constitute the multilayer reflective film, the change with time of the stress of the formed multilayer reflective film can be suppressed, and the changes in the wavelength properties and the reflective properties of the multilayer reflective film to EUV light which is a exposed light can be prevented.
In Patent Document 1, the heating treatment is preferably carried out before forming an absorber layer, since the peak wavelength and the reflectivity of the multilayer reflective film are measured before and after the heating treatment to inspect whether the change of a pattern size formed on a semiconductor substrate due to displacement of matching with a reflective mirror of a pattern transferring device does not substantially result, which is caused by the change of the peak wavelength and the deterioration of the reflectivity due to the difference between each peak length and the reflectivity.
Further, in Patent Document 1, the above-mentioned heating treatment is carried out by contacting the multilayer reflective film formed on a substrate with liquid kept under heating state. Further, in Patent Document 1, as the liquid used for the heating treatment, a cleaning liquid is used, whereby the heating treatment and the cleaning step can be carried out simultaneously.